The invention concerns the manufacture of integrated circuits.
In many integrated circuits, it is necessary to have a clock available. This is the case, for example, with a signal processor which, at all events, requires a clock that provides for the smooth sequencing of the processing operations performed.
The clock may be set up by means of an oscillator internal to the integrated circuit, or else the clock signals may be applied from the outside of the integrated circuit, to a terminal specially reserved for this purpose.
The use of an external clock compells the user of the integrated circuit to provide for this clock, and this indirectly increases the cost of the circuit for the user. Furthermore, it makes it necessary to have a circuit terminal reserved for the clock.
When an oscillator is made on an integrated circuit, the difficulty lies in the precise obtaining of a desired frequency. Indeed, such are the manufacturing variations resulting from the technological processes used that it is not possible to obtain a frequency with sufficient precision. The variation in the natural frequency of oscillation obtained for two identical oscillators coming from the same production line is easily 100% or even more. This results from the fact that the manufacturing processes involve steps for doping, diffusion of impurities at high temperature, deposition of thin insulator layers etc. It is not very easy to achieve full control over the reproducibility of these steps from one circuit to the next.
Moreover, when an internal oscillator is made in the integrated circuit, it is perceived that its frequency varies as a function not only of the variations in manufacture but also of the supply voltage of the circuit. This results from the fact that the frequency of the oscillator depends on the currents that flow through it, and these currents themselves depend on the general supply voltage of the circuit.
Either the oscillation frequency is not of vital importance, and then it is possible to accept an oscillator that is entirely integrated into the circuit, without any external terminal for the reception of a clock signal. Or, on the contrary, the frequency is a vitally important parameter and, in this case, the approach generally used lies in connecting, to the exterior of the integrated circuit, elements (usually resistors or capacitors) for adjusting or controlling the internal oscillator. These adjusting elements have a value that is very precisely known, either because they are not subject to the same manufacturing variations or because they have been selected. However, the drawback is that they must be connected directly to the oscillator of the integrated circuit and, consequently, they make it necessary for the integrated circuit to have additional external pins, specially reserved for this use. Additional external pins are to be avoided to the utmost extent however for they are the biggest factors of cost in integrated circuits.
In a signal processor, namely an electronic circuit capable of carrying out various signal processing tasks under control by instruction, the clock that determines the sequencing of the operations carried out by the microprocessor is a very important circuit element and its frequency has to be well determined. In the signal processing circuits made at present, the clock is external, or it is internal and adjusted by precise external components (quartz elements, resistors, capacitors etc.).